1. Field of the Invention
The present invention relates to the field of switching amplifiers. More specifically, the present invention relates to a novel method of sampling in the signal modulation stage of a digital amplifier to achieve a highly accurate representation of the input signal at substantial gain.
2. Description of the Related Art
There have been various developments pertaining to oversampled, noise-shaping signal processing. These developments have been applicable to both continuous-time (analog) and discrete-time (digital or sampled analog) signals. The constant struggle in this field is to increase the efficiency of the amplifiers. Given the myriad of applications of sound applications in the electronics of today, it is apparent that an efficient audio amplification is highly desirable.
In response to this need, attempts have been made to design switching audio amplifiers using oversampled, noise-shaping modulators, especially delta-sigma modulators. A prior art first order delta-sigma modulator is shown in FIG. 1. A noise shaping network 102 is connected in series with a comparator 104, which is a 1-bit quantizer with sampling rate fs. The output 105 of the comparator is fed back to the noise shaping network via summation element 106. The feedback in turn forces the average value of the quantized output signal to track the average value of the input to the modulator 100. Any difference between the quantized output and modulator input is accumulated in the noise shaping network 102 and eventually corrected. For first-order delta-sigma modulators, noise in the signal band due to quantization error is reduced by approximately 9 dB for each doubling of the oversampling ratio (OSR). The OSR is given by fs/2fo, where 2fo is the Nyquist rate, i.e., twice the bandwidth fo of the baseband signal, and fs is the previously mentioned 1-bit quantizer's sampling rate. For second-order delta-sigma modulators, this noise is reduced by approximately 15 dB (9 dB+6 dB) for the same increase in OSR. However, noise improvements achieved by increases in the OSR, i.e., increases in fs, are ultimately limited as the rise and fall times of the output signal become significant with respect to the sample period.
Accordingly, it would be highly desirable to employ aggressive noise shaping while at the same time maintaining a fixed signal feedback rate for improved noise shaping. This would allow efficient application of audio amplification in many of today's electronics such as multimedia computers.